Air Traffic Control (ATC) is now supplementing voice communications with datalink communications between air traffic controllers and pilots. The datalink communication system is specified worldwide by standards from the International Civil Aviation Organization (ICAO). The datalink communication system uses a communication protocol suite called Aeronautical Telecommunication Network (ATN), which is based on the standard seven layer Open Systems Interconnection (OSI) protocols.
One layer of the ATN is the transport (TPS) layer, which contains a retry timer T1 and logic (algorithm) to manage the retry timer. When the retry timer expires, a datalink message in the form of one Transport Protocol Data Unit (TPDU) or sometimes multiple TPDUs is transmitted again by the transport layer, the value of the retry timer is doubled, and the retry timer is started again. This continues until a TPDU acknowledging the transmitted TPDU is received or retries are exhausted. The foregoing algorithm is defined further in ICAO 9880.
A downlink message, such as a Protected Mode (PM) Controller Pilot Data Link Communications (CPDLC) ATN message, can experience excessive delay when a previous downlink message's TPDUs at the transport layer is still waiting to be acknowledged, retransmissions have not been successful, and the airborne transport layer can only send one TPDU at a time, such as when the transport layer is flow controlled or even when not flow controlled. In these situations, the transport retry time doubles each time the downlink TPDU is retransmitted.
For example, if the initial retry time is 20 seconds and a retransmission occurs, then the retry time doubles to 40 seconds, and if another retransmission occurs, then the retry time doubles again to 80 seconds. The delay incurred by this logic can result in the ground application determining that the downlink message is stale (too old) when it is finally received. This can also contribute to increased round trip delays in messages, including one or more TPDUs waiting for a TPDU in front to be transmitted, since the downlink portion may take a lot longer than expected in this condition.